Health risk assessments are used by many decision makers to determine the potential results of human and ecological exposure to contaminated substances. For example, concerned with potential fertility problems in military personnel, the United States Army has collaborated with the National Institute for Occupational Safety and Health to address the concern. An example of this is a study evaluating Vietnam War veterans who had been exposed to Agent Orange (Operation Ranch Hand). Another study was directed at the exposures of individuals to solvents and fuels during aircraft maintenance duties.
Typically, laboratory test results are combined with computer models that scale up the measured dose-response relationship to the level of the human or the ecological habitat. Field-testing the results of a health risk assessment has heretofore been considered impractical, considering that calculation-based risk assessments ordinarily project the health status of receptors over years or decades of exposure. Consequently, conducting field studies that would corroborate risk assessments would be costly in terms of time and resources.
The current state of practice in screening-level and baseline terrestrial ecological risk assessments (ERA) is the computation of model estimates, such as Hazard Quotients (HQs) for each of the various plants, invertebrates, birds, and mammals that inhabit a particular geographical site of interest. A hazard quotient, the ratio of an animal's estimated daily dietary doses of a chemical to a reputedly safe dose of the same chemical, has notable limitations. A hazard quotient can therefore serve only as a mere risk probability screening tool. Hazard quotients, by themselves, do not indicate that inhabitants at a geographical site of interest are at actual risk.
In most cases, ecological risk assessments only extend to the desktop calculation stage. Although guidance recommends that the field condition be assessed in order to verify the commonplace predictions of moderate to severe impacts accruing to the site biota, rarely do field studies proceed.
For example, ERA modeling is employed to anticipate toxicological effects occurring in the wild (that is, the field) based on a specified HQ level being exceeded. At present, the few field techniques that are available to be applied when working with field organisms, only measure chemical exposures as opposed to health effects. Examples include comparing chemical concentrations in animal and plant tissues of both the chemically-contaminated site and a matched (non-contaminated) reference location.
For example, it can be determined that the spleen of an animal trapped in a contaminated site contains thirty percent more of a toxic chemical than the spleen of an animal trapped at a corresponding reference site (that is, a non-contaminated site). One could wrongly conclude from such measurements, that due to their exposures, inhabitants of the contaminated site are likely to die prematurely or to develop health problems. Such conclusions are unsupported, because tissue concentration cannot be related to health effects. Thus, in the example offered, one would not be able to determine that an animal with thirty percent more of a toxic chemical in its spleen is unable to reproduce or is unable to carry on some other vital biological function. One might suspect that an animal with an excess of a chemical in one of its organs is unhealthy, but the measured chemical level does not prove this.
Similarly, one could measure the amount of a given enzyme present in animals that are exposed to a contaminated site, and also in animals of a noncontaminated reference location. Although animals from the contaminated site may have higher or lower levels of enzyme, there is no way to tell if the animals of the contaminated site are unhealthy.
As described above, although ERA modeling can serve as a risk screening tool, it cannot determine if actual effects are occurring in animals of the contaminated site. All current ERA health assessment measures do not have benchmarks or thresholds for effect, which if exceeded, would tell the risk assessor that effects are in fact occurring.
Thus, there does not currently exist a formal field-truthing methodology for the verification of modeled toxicological effects in terrestrial systems.
Therefore, what is needed is a field-truthing method that is capable of measuring practical toxicological effects in terrestrial systems. Such a method should validate laboratory studies and provide direct assessment of contaminated health impact. Such a method should also have a specific benchmark or threshold with which to measure in order to determine the practical toxicological effect in terrestrial systems.